The present invention relates to an exhaust system for a turbomachine, such as a steam or gas turbine or the like. More specifically, the present invention relates to an exhaust system for axial flow turbomachine in which the flow area of the exhaust housing is locally constricted.
The performance of a steam turbine may generally be improved by lowering the back pressure to which the last row of blades of the turbine is subjected. Consequently, turbines often discharge to a condenser in which a sub-atmospheric pressure is maintained. Typically, the exhaust steam discharging axially from the last row of blades is directed to a condenser mounted below the turbine by turning the flow 90.degree. from the axial to the vertically downward directions. This turning of the flow is accomplished by an exhaust system that includes a diffuser in flow communication with an exhaust housing.
Diffusers are generally comprised of inner and outer flow guides that serve to increase the static pressure by reducing the velocity head. Typically, the cross-sectional shape of the outer flow guide is a simple arcuate shape--see, for example, U.S. Pat. Nos. 3,945,760; 4,863,341; 3,058,720; 3,697,191; and 3,690,786. However, conical shaped diffusers have also been utilized--see, for example, U.S. Pat. No. 4,391,566.
Although outer flow guides are generally of uniform axial length, outer flow guides have been proposed for use in bottom exhaust systems in which the axial length of the outer flow guide varies uniformly around its circumference, being a maximum at the bottom of the diffuser and a minimum at the top--see, U.S. Pat. No. 5,257,906 (Gray et al.), incorporated herein in its entirety by reference. Another outer flow guide that has been used in the past has a constant minimum axial length in the top half of the outer flow guide (that is, in the uppermost 180.degree. of its circumference), a constant maximum length in the lowermost approximately 100.degree. of its circumference, and transition regions at approximately 90.degree.-130.degree. and 230.degree.-270.degree. of its circumference in which the length increases from the minimum to the maximum.
Typically, the exhaust housing receives steam from the diffuser and directs it to the condenser through a bottom outlet opening in the housing. The steam from the diffuser enters the exhaust housing in a 360.degree. arc. However, it discharges from the exhaust housing to the condenser through only the bottom outlet opening. This presents no problem with respect to the steam flowing in the bottom portion of the diffuser since by turning such steam into the radial direction, the diffuser turns the steam directly toward the bottom outlet opening. However, the steam discharging at the top of the diffuser must turn 180.degree. from the vertically upward direction to the vertically downward direction, in addition to turning 90.degree. from the axial direction to the vertically upward direction. As a result of this torturous flow path, losses are experienced by the steam flow that detract from the efficiency of the exhaust system and, therefore, the performance of the turbine.
The outer flow guide serves to minimize these losses by properly guiding the steam flow while turning it from the axial to the radial direction. It is generally thought that in order to properly guide the steam flow, the axial length of the outer flow guide should optimally be equal to at least approximately 50% of the height of the airfoil portions of the last row of blades.
Unfortunately, the inventor has found that in some turbine exhaust systems, especially those of older vintage, the flow area of the exhaust housing is locally constrained--that is, the space from the inlet of the outer flow guide to the exhaust housing is relatively small. As a result, the use of an outer flow guide of the "optimum" length results in insufficient flow area to allow the steam flow to smoothly turn 180.degree. from vertically upward to vertically downward. Consequently, use of an "optimum" length outer flow guide does not result in the optimum thermodynamic performance.
It is therefore desirable to provide a high performance outer flow guide for an exhaust system that turns an axial flow discharging from a turbine into a radial direction, such as vertically downward, and in which the flow area of the exhaust housing is locally constricted.